The present Application claims foreign priority under 35 U.S.C. xc2xa7119(a)-(d)/365(b) based on EP 01301257.0 filed Feb. 14, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a lithographic projection apparatus comprising a radiation system for supplying a projection beam of radiation; a support structure for supporting patterning means, the patterning means serving to pattern the projection beam according to a desired pattern; a substrate table for holding a substrate; and a projection system for projecting the patterned beam onto a target portion of the substrate.
2. Discussion of Related Art
The term xe2x80x9cpatterning componentxe2x80x9d as here employed should be broadly interpreted as referring to a patterning component that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning components include the following.
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing a piezoelectric actuator. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-adressable mirrors. The required matrix addressing can be performed using suitable electronic components. In both of the situations described hereabove, the patterning component can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning component as hereabove set forth.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning component may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatuses, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatuses are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a lithographic projection process it is important to control accurately the dose (i.e. amount of energy per unit area integrated over the duration of the exposure) delivered to the resist. Known resists are designed to have a relatively sharp threshold, whereby the resist is exposed if it receives an amount of energy per unit area above the threshold but remains unexposed if the amount of received energy is less than the threshold. This is used to produce sharp edges in the features in the developed resist, even when diffraction effects cause a gradual tail-off in intensity of the projected images at feature edges. If the beam intensity is substantially incorrect, the exposure intensity profile can cross the resist threshold at the wrong point. Dose control is thus crucial to correct imaging.
In a known lithographic apparatus dose control is done by monitoring the beam intensity at a point in the radiation system and calibrating the absorption of the apparatus between that point and the substrate level. Monitoring the beam intensity is performed using a partially transmissive mirror to divert a known fraction of the projection beam in the radiation system to an energy sensor. The energy sensor measures the energy in the known fraction of the beam and so enables the beam energy at a given point in the radiation system to be determined. The calibration of the absorption of the apparatus, downstream of the partially transmissive mirror, is done by replacing the substrate by an energy sensor for a series of calibration runs. The output of the energy sensor effectively measures variations in the output of the radiation source and is combined with the calibration results of the absorption of the downstream parts of the apparatus to predict the energy level at substrate level. In some cases the prediction of the energy level at substrate level may take account of parameters of the exposure, e.g. radiation system settings. The exposure parameters, e.g. duration or scanning speed, and/or the output of the radiation source can then be adjusted to deliver the desired dose to the resist.
While the known method of dose control takes account of variations in the output of the radiation source and deals well with predictable variations in absorption downstream of the energy sensor, not all variations in absorption are easily or accurately predictable. This is particularly the case for apparatuses using exposure radiation of shorter wavelengths, which are essential to reduce the size of the smallest features that can be imaged, such as 193 nm, 157 nm, or 126 nm. Such wavelengths are heavily absorbed by air and many other gases so that lithographic apparatuses making use of them must be either flushed with non-absorbing gases or evacuated. Any variations in the composition of the flushing gas or leaks from the outside can result in significant and unpredictable variations in the absorption of the beam in the downstream parts of the apparatus and hence of the dose delivered to the resist.
An object of the present invention is therefore to provide an improved dose sensing and control system which avoids or alleviates the problems of known energy sensors and dose control systems.
This and other objects are achieved according to the invention in a lithographic apparatus as specified in the opening paragraph which includes a sensor arranged to detect luminescence radiation produced in said projection system by the passage of said projection beam.
The sensor, which may comprise one or more photodiodes, for example, detects luminescence radiation caused by the interaction of the projection beam radiation, such as ultraviolet radiation, with the material of the projection system, such as calcium fluoride or quartz lens elements. The luminescence radiation intensity is indicative of the dose being delivered to the substrate. Unlike other dose sensors, no part of the projection beam is additionally blocked or diverted, because the luminescence radiation is an intrinsic property of the interaction of the projection beam radiation and the lens. Furthermore, the luminescence radiation can be measured from the projection system very close to the substrate, and therefore, unlike other dose measurements, is not prone to errors due to transmission variations in the optical path from the radiation source to the substrate. The luminescence radiation can be measured at the side of or beyond the end of the projection system and therefore can avoid occupying the very limited space between the projection system and the substrate.
According to one preferred embodiment, the sensor comprises a plurality of detectors. These can capture the luminescence radiation emitted in different directions and can be summed to produce a signal. According to another embodiment, a radiation guide is provided to direct the luminescence radiation emitted in a plurality of directions all to a single detector. This reduces the cost and wiring complexity of a plurality of detectors, and enables the single detector to be located remote from the projection system to reduce the space overhead and to facilitate exchangeability of the detector.
The particular illumination mode and patterning of the projection beam can result in inhomogeneous generation of luminescence radiation. Thus the luminescence radiation in particular directions can vary between different illumination settings and patterns even though the actual dose on the substrate is the same. In either of the above preferred embodiments this problem can be avoided, because the luminescence radiation emitted in a plurality of directions is detected (using a plurality of detectors or using a waveguide arrangement and a single detector), so the resulting signal can be indicative of the actual dose regardless of the particular illumination mode or patterning of the projection beam.
According to a further aspect of the invention there is provided a device manufacturing method comprising the steps of: providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system; using a patterning component to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target area of the layer of radiation-sensitive material on said substrate, and detecting luminescence radiation produced in said projection system by the passage of a projection beam.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms radiation and beam are used to encompass all types of electromagnetic radiation, but specifically ultraviolet radiation, e.g. with a wavelength of 365, 248, 193, 157 or 126 nm.